A belt retractor as is known for example from European patent application 0 456 853 comprises a belt reel taking up the seat belt, a belt tensioning mechanism capable of biasing the belt reel in the winding direction of the seat belt, and a sensor capable of triggering the belt tensioning mechanism and is provided with a safety device translatable by installation of the belt retractor in a vehicle from a secured condition, in which the sensor is blocked, into a released condition in which the sensor is ready to trigger, the safety device including a sensor blocking element which is movable between a blocking position in which the sensor is blocked and a release position in which the sensor is ready to trigger.
A belt tensioning mechanism being triggered by a sensor when required serves to eliminate belt slack from the seat belt system by rotating the belt reel in the winding direction of the seat belt so that a vehicle occupant is involved in the deceleration of the vehicle as early as possible. To prevent triggering of the belt tensioning mechanism during transport of the belt retractor prior to it being mounted in the vehicle, a sensor blocking element is provided which in its blocking position blocks an inertial mass of the sensor. The sensor blocking element is shifted into its release position when the belt retractor is installed in the vehicle. This may be done, for example, by means of a sensing pin which on installation of the belt retractor comes into contact with a part of the vehicle and is urged into the housing of the belt retractor.
The disadvantage in this known belt retractor is that no possibility is provided to prevent an activation of the belt tensioning mechanism when the seat belt is not in use at all. So if the vehicle is involved in an accident, the belt tensioning mechanism is actuated even when, for example, the belt retractor concerned is provided for the front-passenger seat and the front-passenger seat is unoccupied.
The object of the invention is to provide a belt retractor of the aforementioned kind in which a transport safeguarding device, preventing activation of the belt tensioning mechanism when the belt retractor is not incorporated in a vehicle, is combined in a simple way with an occupancy sensor, preventing a triggering of the belt tensioning mechanism when the seat belt of the belt retractor is not in use.